In recent years, electron sources using cathodes with monocrystalline tungsten needle electrodes having coating layers of zirconium and oxygen (hereinafter referred to as ZrO/W electron sources) have been used to obtain electron beams that are brighter and have a longer operating life than hot cathodes (see Non-Patent Document 1).
ZrO/W electron sources are obtained by providing a reservoir consisting of zirconium and oxygen on a monocrystalline tungsten needle cathode having an axial orientation in the <100> orientation, so that zirconium and oxygen are diffused to form a coating layer (hereinafter referred to as a ZrO coating layer), said ZrO coating layer reducing the work function of the (100) plane of the monocrystalline tungsten single crystal from 4.5 eV to about 2.8 eV, so that only the tiny crystalline facet corresponding to the (100) plane formed at the tip of this cathode forms an electron emission region, as a result of which an electron beam that is brighter than that of conventional thermionic cathodes can be obtained, and the operating life is also prolonged.
As shown in FIG. 1, ZrO/W electron sources comprise a needle tip 1 of tungsten in the <100> orientation for emitting an electron beam affixed by welding or the like at a predetermined position on a tungsten filament 3 provided on conductive terminals 4 affixed to insulator 5. A reservoir 2 for zirconium and oxygen is formed at a portion of the needle tip 1.
In equipment using this kind of electron source, assuming that a low-acceleration electron beam is used, the diameter of the electron beam when focused by a lens will be determined by chromatic aberration, and in order to reduce the chromatic aberration, the energy width of the electrons emitted from the electron source must be made smaller.
While lowering the operating temperature of the electron source is effective in reducing chromatic aberration, lowering the operating temperature also dramatically reduces the emission current, so that an electron source with a low work function must be used in order to enable the operational temperature of the electron source to be lowered. In view of the above circumstances, recent years have seen much activity in the search for species adsorbing to monocrystalline tungsten having a low work function, and reservoirs thereof, as an alternative to ZrO adsorption layers.
On the other hand, cold field emission electron sources have needle cathodes of monocrystalline tungsten in the <111>, <310> or <100> orientation welded to a tungsten filament, and have very small emission currents compared with ZrO/W electron sources, but they operate at room temperature and thus have low chromatic aberration and are used as electron sources for high-resolution SEM (see, e.g., Patent Document 1).
Since cold field emission electron sources are operated at room temperature, gas tends to adsorb to the electron emission surface, making the beam current gradually unstable. Therefore, they must be operated in a high degree of vacuum, and must be periodically subjected to a cleaning by heating the filament, called flashing.
More recently, nano-electron sources bearing tiny atomic scale tips have been prepared at the experimental stage, and their application as extremely small point sources for ultrahigh-resolution SEM is being considered. Since the production of nano-electron sources also normally requires a heating step during the production process, they normally have a structure in which a needle tip similar to that of FIG. 1 is welded to a heating filament (see Patent Document 2).    Patent Document 1: JP S50-11576A    Patent Document 2: JP 2006-134638A    Non-Patent Document 1: D. Tuggle, J. Vac. Sci. Technol., 16, p. 1699 (1979).